Method of hardening metal articles by nitriding



Patented Jan. 31, 1933 UNITED STATES PATENT, IOFFIICE PIERRE FRANQOIS MARIE AUBERT, OF BOULOGNE-SUR-SEINE, ADRIEN J'ULES PIERRE DUVAL, OF PARIS, AND HENRI AUGUSTE MARGUERITE DUVAL, OF N EUILLY-SUR- SEINE, FRANCE, ASSIQNORS TO THE N ITRALLOY CORPORATION, A CORPORATION OI DELAWARE METHOD OF EARDENING METAL ARTICLES BY NITRIDING Ho Drawing. Original application iiled October 18,1927, serial No. 227,047. Divided and this application I filed Kay 5, 1981, Serial No. 535,290,and,1n Germany May 10, 1927.

This invention relates to the surface hardening of ferrous metal articles by the action of nitrogen-containing compounds, and has particular reference to a process of nltridlng certain selected portions of such artlcles while protecting certain other portions from penetration by nitrogen during the hardening operation.

According to one embodiment of the 1 nvention disclosed in our application, Serial N 0. 227,047, filed -October 1 8, 1927, of which this application is a divislon, the portlons to be protected during the hardening operation are coated with a metal or alloy fusible at or below the nitriding temperature ran e, e. g. tin or a tin alloy. While such fusl 1e metals or alloys have the property of effectively preventing nitriding of areas to wh ch they have been applied and the protection persists even after the article so coated has been subjected to nitriding temperatures for long periods and for repeated nitridin operations, nevertheless there is the disa vantage that when articles so coated are heated to the nitriding temperature portions of the coating metal will, unless restrained from so doing, flow over the surfaces thatare to be nitrided and as a consequence prevent mtriding of areas where it is intended that the nitride hardening shall be produced. In the prior application above mentioned, we have described and claimed one method for overcoming this difliculty, namely, superimposing upon such coated areas a, second coating of a material infusible at the nitridlng temperature.

The present invention has for an object to inhibit the undesired efi'ects above mentioned by so treating the uncoated surfaces, 1. e. those that are to be hardened bythe nltridlng treatment, as to make-it impossible for portions of the fusible metal to adhere to them. We have found that the undesired coating and protection of the surfaces to be nitrided can be inhibited by suitably treating such surfaces so as to make it impossible for the tin allo or other easily fusible metal to adhere to t em without at the same time preventing them in any way from acquiring the nitride hardness. More specifically, we have found containing atmosphere.

of producing such slight oxidation consists in dipping the selectively coated articles in a bath of fused salts, e. g. a bath of sodium nitrate or potassium nitrate, heated to about 400 C. In this bath, the excess tin or other fusible metal coating, i. e. that portion which does not remain to protect during the subsequent nitriding operation, becomes liquid and flows off without being able to adhere to the uncoated portions. In other words, the salt bath treatment renders the uncoated surface unsusceptible of wetting by tin, tin alloys or similar easily fusible metals. Moreover, the tin drops or globules cover themselves with an oxide layer so that they are unable to exert a'tinning effect.

The slight and controlled oxidation that is produced by this treatment is not to be confused with the oxidation that is produced when the metal is heated for prolonged periods at high temperatures in an oxygen- In the former case only an extremely thin oxide film is formed and this is quickly reduced without untoward effect on the nitriding operation by the hydrogen liberated from the ammonia used in the nitriding operation. In the latter case a relatively thick layer of oxide or scale is produced, which would need to be removed before the article could be nitrided satisfactorily.

After having been treated in the indicated manner, the pieces are subjected to the nitriding treatment during which the untinned surfaces acquire the known nitride hardness, while the previously tinned surfaces remain unaffected and may be machined without difliculty.

We have not positively determined why the protective coating persists tures above the melting points of tin or the tin alloys used. While we do not intend to be bound by any theory advanced, this may be due either to the formation of an ex-v at temperatremely thin layer of the tin or tin alloy which is not driven off at the, nitriding temperature or only after prolonged and repeated exposure to such temperature. When a portion that has been previously tinned is to be hardened subsequently, the surface'layer must be removed down to a certain depth.

The present invention is particularly applicable for use in conjunction with the nitriding process described and claimed in U. S. patent to Fry No. 1,487,554, although it is to be understood that it can be used in conjunction with other processes for imparting nitride hardness that may be developed.

We claim 1. In the hardening by nitriding of selected portions of a metal article, the steps which comprise applyin on the portions to be protected a coating 0 a metallic composition incapable of substantial penetration by nitrogen at a nitriding temperature and fusible at such temperature, coated portions of said article to render them unsusceptible of wetting by said metallic composition when the same is brought in a liquid state into contact therewith, and thereafter subjecting said article to a nitriding treatment.

' 2. In the hardening by nitriding of select- I ed portions of a metal article, the steps which comprise applying a coating of tin or a tin "alloy to the portions to be protected, im-

mersing said coated portions in a heated oxygen-containing fused salt bath to slightly oxidize the uncoated portions of said article to render them unsusceptible of wetting by tin or; a tin alloy, and thereafter subjecting said article to a nitriding treatment.

3. In the hardening by nitriding of selected portions of a metal article, the steps which comprise applying a coating of tin or a tin alloy to the portions to be protected, immersing said coated portions in a heatedbath of a nitrate ofa metal of the alkali metal group, and thereafter subjecting said article to a nitriding treatment.

4. In the hardening by nitridin lected portions of a metal article, t e steps which comprise applying a coating of tin or a tin alloy to the portions to be protected, im-

mersing said coated portions in a heated bath of sodium mtrate, and thereafter subjecting said article to a nltriding treatment.

In testimony whereof, we have signed our names to this specification this 22 day of April, 1931. i

' PIERRE FRANQOIS MARIE AUBERT.

ADRIEN JULES PIERRE DUVAL. HENRI AUGUSTE HARGUERITE DUVAL slightly oxidizing the un- 

